Apache Module mod_rewrite

Summary

The mod_rewrite module uses a rule-based rewriting
engine, based on a PCRE regular-expression parser, to rewrite requested URLs on
the fly. By default, mod_rewrite maps a URL to a filesystem
path. However, it can also be used to redirect one URL to another URL, or
to invoke an internal proxy fetch.

mod_rewrite provides a flexible and powerful way to
manipulate URLs using an unlimited number of rules. Each rule can have an
unlimited number of attached rule conditions, to allow you to rewrite URL
based on server variables, environment variables, HTTP headers, or time
stamps.

mod_rewrite operates on the full URL path, including the
path-info section. A rewrite rule can be invoked in
httpd.conf or in .htaccess. The path generated
by a rewrite rule can include a query string, or can lead to internal
sub-processing, external request redirection, or internal proxy
throughput.

Directives

Topics

mod_rewrite offers detailed logging of its actions
at the trace1 to trace8 log levels. The
log level can be set specifically for mod_rewrite
using the LogLevel directive: Up to
level debug, no actions are logged, while trace8
means that practically all actions are logged.

Using a high trace log level for mod_rewrite
will slow down your Apache HTTP Server dramatically! Use a log
level higher than trace2 only for debugging!

Example

LogLevel alert rewrite:trace3

RewriteLog

Those familiar with earlier versions of
mod_rewrite will no doubt be looking for the
RewriteLog and RewriteLogLevel
directives. This functionality has been completely replaced by the
new per-module logging configuration mentioned above.

To get just the mod_rewrite-specific log
messages, pipe the log file through grep:

The RewriteBase directive specifies the
URL prefix to be used for per-directory (htaccess)
RewriteRule directives that substitute a relative
path.

This directive is required when you use a relative path
in a substitution in per-directory (htaccess) context unless either
of the following conditions are true:

The original request, and the substitution, are underneath the
DocumentRoot
(as opposed to reachable by other means, such as
Alias).

The filesystem path to the directory containing the
RewriteRule, suffixed by the relative
substitution is also valid as a URL path on the server
(this is rare).

In Apache HTTP Server 2.4.13 and later, this directive may be
omitted when the request is mapped via
Alias
or mod_userdir.

In the example below, RewriteBase is necessary
to avoid rewriting to http://example.com/opt/myapp-1.2.3/welcome.html
since the resource was not relative to the document root. This
misconfiguration would normally cause the server to look for an "opt"
directory under the document root.

The RewriteCond directive defines a
rule condition. One or more RewriteCond
can precede a RewriteRule
directive. The following rule is then only used if both
the current state of the URI matches its pattern, and if these conditions are met.

TestString is a string which can contain the
following expanded constructs in addition to plain text:

RewriteRule backreferences: These are
backreferences of the form $N
(0 <= N <= 9). $1 to $9 provide access to the grouped
parts (in parentheses) of the pattern, from the
RewriteRule which is subject to the current
set of RewriteCond conditions. $0 provides
access to the whole string matched by that pattern.

RewriteCond backreferences: These are
backreferences of the form %N
(0 <= N <= 9). %1 to %9 provide access to the grouped
parts (again, in parentheses) of the pattern, from the last matched
RewriteCond in the current set
of conditions. %0 provides access to the whole string matched by
that pattern.

These variables all
correspond to the similarly named HTTP
MIME-headers, C variables of the Apache HTTP Server or
struct tm fields of the Unix system.
Most are documented here
or elsewhere in the Manual or in the CGI specification.

This is the version of the Apache httpd module API
(the internal interface between server and
module) in the current httpd build, as defined in
include/ap_mmn.h. The module API version
corresponds to the version of Apache httpd in use (in
the release version of Apache httpd 1.3.14, for
instance, it is 19990320:10), but is mainly of
interest to module authors.

CONN_REMOTE_ADDR

Since 2.4.8: The peer IP address of the connection (see the
mod_remoteip module).

HTTPS

Will contain the text "on" if the connection is
using SSL/TLS, or "off" otherwise. (This variable
can be safely used regardless of whether or not
mod_ssl is loaded).

IS_SUBREQ

Will contain the text "true" if the request
currently being processed is a sub-request,
"false" otherwise. Sub-requests may be generated
by modules that need to resolve additional files
or URIs in order to complete their tasks.

The full local filesystem path to the file or
script matching the request, if this has already
been determined by the server at the time
REQUEST_FILENAME is referenced. Otherwise,
such as when used in virtual host context, the same
value as REQUEST_URI. Depending on the value of
AcceptPathInfo, the
server may have only used some leading components of the
REQUEST_URI to map the request to a file.

REQUEST_SCHEME

Will contain the scheme of the request (usually
"http" or "https"). This value can be influenced with
ServerName.

REQUEST_URI

The path component of the requested URI,
such as "/index.html". This notably excludes the
query string which is available as as its own variable
named QUERY_STRING.

THE_REQUEST

The full HTTP request line sent by the
browser to the server (e.g., "GET
/index.html HTTP/1.1"). This does not
include any additional headers sent by the
browser. This value has not been unescaped
(decoded), unlike most other variables below.

If the TestString has the special value expr,
the CondPattern will be treated as an
ap_expr. HTTP headers referenced in the
expression will be added to the Vary header if the novary
flag is not given.

Other things you should be aware of:

The variables SCRIPT_FILENAME and REQUEST_FILENAME
contain the same value - the value of the
filename field of the internal
request_rec structure of the Apache HTTP Server.
The first name is the commonly known CGI variable name
while the second is the appropriate counterpart of
REQUEST_URI (which contains the value of the
uri field of request_rec).

If a substitution occurred and the rewriting continues,
the value of both variables will be updated accordingly.

If used in per-server context (i.e., before the
request is mapped to the filesystem) SCRIPT_FILENAME and
REQUEST_FILENAME cannot contain the full local filesystem
path since the path is unknown at this stage of processing.
Both variables will initially contain the value of REQUEST_URI
in that case. In order to obtain the full local filesystem
path of the request in per-server context, use an URL-based
look-ahead %{LA-U:REQUEST_FILENAME} to determine
the final value of REQUEST_FILENAME.

%{ENV:variable}, where variable can be
any environment variable, is also available.
This is looked-up via internal
Apache httpd structures and (if not found there) via
getenv() from the Apache httpd server process.

%{SSL:variable}, where variable is the
name of an SSL environment
variable, can be used whether or not
mod_ssl is loaded, but will always expand to
the empty string if it is not. Example:
%{SSL:SSL_CIPHER_USEKEYSIZE} may expand to
128.

%{HTTP:header}, where header can be
any HTTP MIME-header name, can always be used to obtain the
value of a header sent in the HTTP request.
Example: %{HTTP:Proxy-Connection} is
the value of the HTTP header
``Proxy-Connection:''.

If a HTTP header is used in a condition this header is added to
the Vary header of the response in case the condition evaluates
to true for the request. It is not added if the
condition evaluates to false for the request. Adding the HTTP header
to the Vary header of the response is needed for proper caching.

It has to be kept in mind that conditions follow a short circuit
logic in the case of the 'ornext|OR' flag
so that certain conditions might not be evaluated at all.

%{LA-U:variable}
can be used for look-aheads which perform
an internal (URL-based) sub-request to determine the final
value of variable. This can be used to access
variable for rewriting which is not available at the current
stage, but will be set in a later phase.

For instance, to rewrite according to the
REMOTE_USER variable from within the
per-server context (httpd.conf file) you must
use %{LA-U:REMOTE_USER} - this
variable is set by the authorization phases, which come
after the URL translation phase (during which mod_rewrite
operates).

On the other hand, because mod_rewrite implements
its per-directory context (.htaccess file) via
the Fixup phase of the API and because the authorization
phases come before this phase, you just can use
%{REMOTE_USER} in that context.

%{LA-F:variable} can be used to perform an internal
(filename-based) sub-request, to determine the final value
of variable. Most of the time, this is the same as
LA-U above.

CondPattern is the condition pattern,
a regular expression which is applied to the
current instance of the TestString.
TestString is first evaluated, before being matched against
CondPattern.

CondPattern is usually a
perl compatible regular expression, but there is
additional syntax available to perform other useful tests against
the Teststring:

You can prefix the pattern string with a
'!' character (exclamation mark) to negate the result
of the condition, no matter what kind of CondPattern is used.

You can perform lexicographical string comparisons:

'<CondPattern' (lexicographically
precedes)
Treats the CondPattern as a plain string and
compares it lexicographically to TestString. True if
TestString lexicographically precedes
CondPattern.

'>CondPattern' (lexicographically
follows)
Treats the CondPattern as a plain string and
compares it lexicographically to TestString. True if
TestString lexicographically follows
CondPattern.

'=CondPattern' (lexicographically
equal)
Treats the CondPattern as a plain string and
compares it lexicographically to TestString. True if
TestString is lexicographically equal to
CondPattern (the two strings are exactly
equal, character for character). If CondPattern
is "" (two quotation marks) this
compares TestString to the empty string.

'<=CondPattern' (lexicographically
less than or equal to)
Treats the CondPattern as a plain string and
compares it lexicographically to TestString. True
if TestString lexicographically precedes
CondPattern, or is equal to CondPattern
(the two strings are equal, character for character).

'>=CondPattern' (lexicographically
greater than or equal to)
Treats the CondPattern as a plain string and
compares it lexicographically to TestString. True
if TestString lexicographically follows
CondPattern, or is equal to CondPattern
(the two strings are equal, character for character).

You can perform integer comparisons:

'-eq' (is numerically
equal to)
The TestString is treated as an integer, and is
numerically compared to the CondPattern. True if
the two are numerically equal.

'-ge' (is numerically
greater than or equal to)
The TestString is treated as an integer, and is
numerically compared to the CondPattern. True if
the TestString is numerically greater than or equal
to the CondPattern.

'-gt' (is numerically
greater than)
The TestString is treated as an integer, and is
numerically compared to the CondPattern. True if
the TestString is numerically greater than
the CondPattern.

'-le' (is numerically
less than or equal to)
The TestString is treated as an integer, and is
numerically compared to the CondPattern. True if
the TestString is numerically less than or equal
to the CondPattern. Avoid confusion with the
-l by using the -L or
-h variant.

'-lt' (is numerically
less than)
The TestString is treated as an integer, and is
numerically compared to the CondPattern. True if
the TestString is numerically less than
the CondPattern. Avoid confusion with the
-l by using the -L or
-h variant.

You can perform various file attribute tests:

'-d' (is
directory)
Treats the TestString as a pathname and tests
whether or not it exists, and is a directory.

'-f' (is regular
file)
Treats the TestString as a pathname and tests
whether or not it exists, and is a regular file.

'-F' (is existing file, via
subrequest)
Checks whether or not TestString is a valid file,
accessible via all the server's currently-configured
access controls for that path. This uses an internal
subrequest to do the check, so use it with care -
it can impact your server's performance!

'-H' (is symbolic link, bash convention)
See -l.

'-l' (is symbolic
link)
Treats the TestString as a pathname and tests
whether or not it exists, and is a symbolic link. May also
use the bash convention of -L or
-h if there's a possibility of confusion
such as when using the -lt or
-le tests.

'-L' (is symbolic link, bash convention)
See -l.

'-s' (is regular file, with
size)
Treats the TestString as a pathname and tests
whether or not it exists, and is a regular file with size greater
than zero.

'-U' (is existing URL, via
subrequest)
Checks whether or not TestString is a valid URL,
accessible via all the server's currently-configured
access controls for that path. This uses an internal
subrequest to do the check, so use it with care -
it can impact your server's performance!

This flag only returns information about things
like access control, authentication, and authorization. This flag
does not return information about the status code the
configured handler (static file, CGI, proxy, etc.) would have
returned.

'-x' (has executable
permissions)
Treats the TestString as a pathname and tests
whether or not it exists, and has executable permissions.
These permissions are determined according to
the underlying OS.

If the TestString has the special value expr, the
CondPattern will be treated as an
ap_expr.

In the below example, -strmatch is used to
compare the REFERER against the site hostname,
to block unwanted hotlinking.

You can also set special flags for
CondPattern by appending
[flags]
as the third argument to the RewriteCond
directive, where flags is a comma-separated list of any of the
following flags:

'nocase|NC'
(no case)
This makes the test case-insensitive - differences
between 'A-Z' and 'a-z' are ignored, both in the
expanded TestString and the CondPattern.
This flag is effective only for comparisons between
TestString and CondPattern. It has no
effect on filesystem and subrequest checks.

'ornext|OR'
(or next condition)
Use this to combine rule conditions with a local OR
instead of the implicit AND. Typical example:

Without this flag you would have to write the condition/rule
pair three times.

'novary|NV'
(no vary)
If a HTTP header is used in the condition, this flag prevents
this header from being added to the Vary header of the response.
Using this flag might break proper caching of the response if
the representation of this response varies on the value of this header.
So this flag should be only used if the meaning of the Vary header
is well understood.

Example:

To rewrite the Homepage of a site according to the
``User-Agent:'' header of the request, you can
use the following:

Explanation: If you use a browser which identifies itself
as a mobile browser (note that the example is incomplete, as
there are many other mobile platforms), the mobile version of
the homepage is served. Otherwise, the standard page is served.

The RewriteEngine directive enables or
disables the runtime rewriting engine. If it is set to
off this module does no runtime processing at
all. It does not even update the SCRIPT_URx
environment variables.

Use this directive to disable rules in a particular context,
rather than commenting out all the RewriteRule directives.

Note that rewrite configurations are not
inherited by virtual hosts. This means that you need to have a
RewriteEngine on directive for each virtual host
in which you wish to use rewrite rules.

RewriteMap directives of the type prg
are not started during server initialization if they're defined in a
context that does not have RewriteEngine set to
on

The RewriteMap directive defines a
Rewriting Map which can be used inside rule
substitution strings by the mapping-functions to
insert/substitute fields through a key lookup. The source of
this lookup can be of various types.

The MapName is
the name of the map and will be used to specify a
mapping-function for the substitution strings of a rewriting
rule via one of the following constructs:

${MapName:LookupKey}${MapName:LookupKey|DefaultValue}

When such a construct occurs, the map MapName is
consulted and the key LookupKey is looked-up. If the
key is found, the map-function construct is substituted by
SubstValue. If the key is not found then it is
substituted by DefaultValue or by the empty string
if no DefaultValue was specified. Empty values
behave as if the key was absent, therefore it is not possible
to distinguish between empty-valued keys and absent keys.

The RewriteOptions directive sets some
special options for the current per-server or per-directory
configuration. The Option string can currently
only be one of the following:

Inherit

This forces the current configuration to inherit the
configuration of the parent. In per-virtual-server context,
this means that the maps, conditions and rules of the main
server are inherited. In per-directory context this means
that conditions and rules of the parent directory's
.htaccess configuration or
<Directory>
sections are inherited. The inherited rules are virtually copied
to the section where this directive is being used. If used in
combination with local rules, the inherited rules are copied behind
the local rules. The position of this directive - below or above
of local rules - has no influence on this behavior. If local
rules forced the rewriting to stop, the inherited rules won't
be processed.

Rules inherited from the parent scope are applied
after rules specified in the child scope.

InheritBefore

Like Inherit above, but the rules from the parent scope
are applied before rules specified in the child scope.
Available in Apache HTTP Server 2.3.10 and later.

InheritDown

If this option is enabled, all child configurations will inherit
the configuration of the current configuration. It is equivalent to
specifying RewriteOptions Inherit in all child
configurations. See the Inherit option for more details
on how the parent-child relationships are handled.
Available in Apache HTTP Server 2.4.8 and later.

InheritDownBefore

Like InheritDown above, but the rules from the current
scope are applied before rules specified in any child's
scope.
Available in Apache HTTP Server 2.4.8 and later.

IgnoreInherit

This option forces the current and child configurations to ignore
all rules that would be inherited from a parent specifying
InheritDown or InheritDownBefore.
Available in Apache HTTP Server 2.4.8 and later.

AllowNoSlash

By default, mod_rewrite will ignore URLs that map to a
directory on disk but lack a trailing slash, in the expectation that
the mod_dir module will issue the client with a redirect to
the canonical URL with a trailing slash.

When the DirectorySlash directive
is set to off, the AllowNoSlash option can be enabled to ensure
that rewrite rules are no longer ignored. This option makes it possible to
apply rewrite rules within .htaccess files that match the directory without
a trailing slash, if so desired.
Available in Apache HTTP Server 2.4.0 and later.

AllowAnyURI

When RewriteRule
is used in VirtualHost or server context with
version 2.2.22 or later of httpd, mod_rewrite
will only process the rewrite rules if the request URI is a URL-path. This avoids
some security issues where particular rules could allow
"surprising" pattern expansions (see CVE-2011-3368
and CVE-2011-4317).
To lift the restriction on matching a URL-path, the
AllowAnyURI option can be enabled, and
mod_rewrite will apply the rule set to any
request URI string, regardless of whether that string matches
the URL-path grammar required by the HTTP specification.
Available in Apache HTTP Server 2.4.3 and later.

Security Warning

Enabling this option will make the server vulnerable to
security issues if used with rewrite rules which are not
carefully authored. It is strongly recommended
that this option is not used. In particular, beware of input
strings containing the '@' character which could
change the interpretation of the transformed URI, as per the
above CVE names.

MergeBase

With this option, the value of RewriteBase is copied from where it's explicitly defined
into any sub-directory or sub-location that doesn't define its own
RewriteBase. This was the
default behavior in 2.4.0 through 2.4.3, and the flag to restore it is
available Apache HTTP Server 2.4.4 and later.

IgnoreContextInfo

In versions 2.4.13 and later, when a relative substitution is made
in directory (htaccess) context and RewriteBase has not been set, this module uses some
extended URL and filesystem context information to change the
relative substitution back into a URL. Modules such as
mod_userdir and mod_alias
supply this extended context info. This option disables the behavior
introduced in 2.4.13 and should only be set if all of the conditions
above are present and a substituion has an unexpected result.

The RewriteRule directive is the real
rewriting workhorse. The directive can occur more than once,
with each instance defining a single rewrite rule. The
order in which these rules are defined is important - this is the order
in which they will be applied at run-time.

Pattern is
a perl compatible regular
expression. On the first RewriteRule, it is matched against
the (%-decoded) URL-path (or
file-path, depending
on the context) of the request. Subsequent patterns are matched against the
output of the last matching RewriteRule.

In VirtualHost context,
The Pattern will initially be matched against the part of the
URL after the hostname and port, and before the query string (e.g. "/app1/index.html").

In Directory and htaccess context,
the Pattern will initially be matched against the
filesystem path, after removing the prefix that led the server
to the current RewriteRule (e.g. "app1/index.html"
or "index.html" depending on where the directives are defined).

If you wish to match against the hostname, port, or query string, use a
RewriteCond with the
%{HTTP_HOST}, %{SERVER_PORT}, or
%{QUERY_STRING} variables respectively.

Per-directory Rewrites

The rewrite engine may be used in .htaccess files and in <Directory> sections, with some additional
complexity.

To enable the rewrite engine in this context, you need to set
"RewriteEngine On" and
"Options FollowSymLinks" must be enabled. If your
administrator has disabled override of FollowSymLinks for
a user's directory, then you cannot use the rewrite engine. This
restriction is required for security reasons.

When using the rewrite engine in .htaccess files the
per-directory prefix (which always is the same for a specific
directory) is automatically removed for the RewriteRule pattern matching
and automatically added after any relative (not starting with a
slash or protocol name) substitution encounters the end of a rule set.
See the RewriteBase
directive for more information regarding what prefix will be added back to
relative substitutions.

If you wish to match against the full URL-path in a per-directory
(htaccess) RewriteRule, use the %{REQUEST_URI} variable in
a RewriteCond.

The removed prefix always ends with a slash, meaning the matching occurs against a string which
never has a leading slash. Therefore, a Pattern with ^/ never
matches in per-directory context.

Although rewrite rules are syntactically permitted in <Location> and <Files> sections
(including their regular expression counterparts), this
should never be necessary and is unsupported. A likely feature
to break in these contexts is relative substitutions.

In mod_rewrite, the NOT character
('!') is also available as a possible pattern
prefix. This enables you to negate a pattern; to say, for instance:
``if the current URL does NOT match this
pattern''. This can be used for exceptional cases, where
it is easier to match the negative pattern, or as a last
default rule.

Note

When using the NOT character to negate a pattern, you cannot include
grouped wildcard parts in that pattern. This is because, when the
pattern does NOT match (ie, the negation matches), there are no
contents for the groups. Thus, if negated patterns are used, you
cannot use $N in the substitution string!

The Substitution of a
rewrite rule is the string that replaces the original URL-path that
was matched by Pattern. The Substitution may
be a:

file-system path

Designates the location on the file-system of the resource
to be delivered to the client. Substitutions are only
treated as a file-system path when the rule is configured in
server (virtualhost) context and the first component of the
path in the substitution exists in the file-system

URL-path

A DocumentRoot-relative path to the
resource to be served. Note that mod_rewrite
tries to guess whether you have specified a file-system path
or a URL-path by checking to see if the first segment of the
path exists at the root of the file-system. For example, if
you specify a Substitution string of
/www/file.html, then this will be treated as a
URL-path unless a directory named www
exists at the root or your file-system (or, in the case of
using rewrites in a .htaccess file, relative to
your document root), in which case it will
be treated as a file-system path. If you wish other
URL-mapping directives (such as Alias) to be applied to the
resulting URL-path, use the [PT] flag as
described below.

Absolute URL

If an absolute URL is specified,
mod_rewrite checks to see whether the
hostname matches the current host. If it does, the scheme and
hostname are stripped out and the resulting path is treated as
a URL-path. Otherwise, an external redirect is performed for
the given URL. To force an external redirect back to the
current host, see the [R] flag below.

- (dash)

A dash indicates that no substitution should be performed
(the existing path is passed through untouched). This is used
when a flag (see below) needs to be applied without changing
the path.

Back-references are identifiers of the form
$N
(N=0..9), which will be replaced
by the contents of the Nth group of the
matched Pattern. The server-variables are the same
as for the TestString of a RewriteCond
directive. The mapping-functions come from the
RewriteMap directive and are explained there.
These three types of variables are expanded in the order above.

Rewrite rules are applied to the results of previous rewrite
rules, in the order in which they are defined
in the config file. The URL-path or file-system path (see "What is matched?", above) is completely
replaced by the Substitution and the
rewriting process continues until all rules have been applied,
or it is explicitly terminated by an
L flag,
or other flag which implies immediate termination, such as
END or
F.

Modifying the Query String

By default, the query string is passed through unchanged. You
can, however, create URLs in the substitution string containing
a query string part. Simply use a question mark inside the
substitution string to indicate that the following text should
be re-injected into the query string. When you want to erase an
existing query string, end the substitution string with just a
question mark. To combine new and old query strings, use the
[QSA] flag.

Additionally you can set special actions to be performed by
appending [flags]
as the third argument to the RewriteRule
directive. Flags is a comma-separated list, surround by square
brackets, of any of the flags in the following table. More
details, and examples, for each flag, are available in the Rewrite Flags document.

Rule is chained to the following rule. If the rule fails,
the rule(s) chained to it will be skipped. details ...

cookie|CO=NAME:VAL

Sets a cookie in the client browser. Full syntax is:
CO=NAME:VAL:domain[:lifetime[:path[:secure[:httponly]]]] details ...

discardpath|DPI

Causes the PATH_INFO portion of the rewritten URI to be
discarded. details
...

END

Stop the rewriting process immediately and don't apply any
more rules. Also prevents further execution of rewrite rules
in per-directory and .htaccess context. (Available in 2.3.9 and later)
details ...

env|E=[!]VAR[:VAL]

Causes an environment variable VAR to be set (to the
value VAL if provided). The form !VAR causes
the environment variable VAR to be unset.
details ...

Home directory expansion

When the substitution string begins with a string
resembling "/~user" (via explicit text or backreferences), mod_rewrite performs
home directory expansion independent of the presence or configuration
of mod_userdir.

This expansion does not occur when the PT
flag is used on the RewriteRule
directive.

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